restore: use /proc/self/mountinfo for collecting mounts fo the root task (v3)

If the root task is forked in a new pidns, it can't use its pid for
accessing /proc, because this proc belongs to the source pidns.

v2: don't copy a static string.
v3: take a bright part of Tycho's patch

Reported-by: Tycho Andersen <tycho.andersen@canonical.com>
Cc: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Acked-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>